EX UPGRADE and CARDS standard wavetable can be expanded to EX with further 4 Mb PCM, in this case the rackmount became M1EX (EX upgrade is factory native in successive "T" series).
Even more waveforms can be added using Korg M1 PCM cards or others AI2 compatible Korg systems like Korg M3r or Korg Wavestation. Here's the list of the official cards (card picture from ebay auctions) but there are plenty more from third-part :

Code

Title

Demo

PIC

MSC-01S; MPC-01S

Variety

MSC-02S; MPC-02S

Synth 1

MSC-03S; MPC-03S

Drums 1

MSC-04S; MPC-04S

Orchestra

MSC-05S; MPC-05S

Piano

MSC-06S; MPC-06S

Fretted

MSC-07S; MPC-07S

Synth 2

MSC-08S; MPC-08S

Percussion

MSC-09S; MPC-09S

Organ

MSC-10S; MPC-10S

Ethnic 1

MSC-11S; MPC-11S

Brass

MSC-12S; MPC-12S

Synth 3

MSC-13S; MPC-13S

Ethnic 2

MSC-14S; MPC-14S

Sound Effects

MSC-15S; MPC-15S

Drums 2

MSC-16S; MPC-16S

Environment

PERFORMANCE the M1R is a multitimbric module with special COMBI mode which manages up to 8 parts in the same time with individual parameter settings: